Catalyst and sorbent material for the production of hydrogen

ABSTRACT

A catalyst and sorbent is disclosed which comprises pellets with an absorbent core and a protective shell with a catalyst in the shell. Such material is especially well suited for steam reforming of hydrocarbons to produce hydrogen since a reforming catalyst can be incorporated in the shell and a sorbent for the by-product carbon dioxide can be used for the core. It is also well suited for producing hydrogen from carbon monoxide by means of the water gas shift reaction. The shell can be made sufficiently strong and durable for moving bed applications as well as fixed bed applications.

PRIORITY CLAIM

This application is a Divisional of U.S. patent application Ser. No.10/218,803 filed Aug. 14, 2002, which application claims priority toU.S. Provisional Application Ser. No. 60/312,529 filed Aug. 15, 2001 andis incorporated by reference in its entirety.

FIELD OF THE INVENTION

This invention relates to a pelletized material having an absorbent coreencased in a strong but porous shell having catalytic properties. Such amaterial is preferred for industrial chemical processes which producehydrogen and carbon dioxide. These processes include the steam reformingof hydrocarbons and processes based on the water gas shift reaction.

BACKGROUND OF THE INVENTION

Hydrogen is an important raw material in the chemical and petroleumindustries. Large quantities are used in the manufacture of ammonia andmethanol and in a variety of petroleum hydrotreating processes.

Making hydrogen from methane is of particular interest because in thefuture hydrogen will be used for the generation of electric power byemploying highly efficient fuel cells. While methane is the principalcomponent of natural gas and may be plentiful, it can also be producedby hydrogasification of coal.

Currently, the primary method for converting methane and other lighthydrocarbons to produce hydrogen is based on steam reforming. The steammethane reforming (SMR) process often involves multiple steps and severeoperating conditions, including high temperatures and pressures.

A proposed method for improving the efficiency of steam methanereforming (SMR) is the Sorption Enhanced Reaction Process (SERP). TheSERP method uses a fixed packed bed of an admixture of an SMR catalystand a chemisorbent to remove carbon dioxide selectively from thereaction zone. The SERP process allows for the use of lower temperaturesthen those utilized in conventional SMR methods, and provides a higherdegree of purity of the resulting hydrogen product.

Within the last few years, the concept of combining reaction andseparation steps to simplify various chemical processes, conserveenergy, and/or to improve product quality and yield has becomeeconomically attractive. Reactive distillation is one method that hasbeen recently commercialized, along with membrane reactors.Membrane-based reaction systems may involve the use of metallicmembranes which only small molecules like hydrogen can permeate, orpolymeric, ceramic, and zeolitic membranes. The membranes may act aspermselective barriers, or as an integral part of the catalyticallyactive surface.

The present inventors have now discovered a unique method of producinghydrogen through the reaction of steam with methane, other lighthydrocarbons, or carbon monoxide using a catalyst and sorbent combinedin the same pelletized material. The method is unique compared topresently available technology in that it does not require the catalyticreforming and product separation steps to be conducted with differentmaterials in completely separate steps.

It is therefore a primary objective of the claimed invention to providea material that is capable of converting methane, other lighthydrocarbons, or carbon monoxide to hydrogen and at the same timeseparate the hydrogen from carbon dioxide co-product.

It is a further objective of the present invention to provide a one-stepmethod of producing hydrogen through the conversion of methane, otherlight hydrocarbons, or carbon monoxide using a singular material.

It is a further objective of the present invention to provide a materialthat is regenerable.

It is a further objective of the present invention to provide a materialthat includes a catalyst to enhance and promote the conversion ofmethane to hydrogen.

It is yet a further objective of the present invention to provide amaterial that is durable and attrition resistant.

It is still a further objective of the present invention to provide amaterial that is economical to manufacture and use.

These and other objectives will become apparent after review of thefollowing description and claims of the invention which follow.

SUMMARY OF THE INVENTION

The invention describes a catalyst and sorbent in pellet form whereineach pellet combines a reactive core with a porous protective shellhaving a catalyst embedded or coated on the surface of the pores of theshell. Such materials are useful for various hydrogen manufacturingprocesses, and specifically for catalytically reforming methane or otherhydrocarbon gases to produce hydrogen and carbon dioxide while at thesame time separating the hydrogen from the carbon dioxide.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The core-in-shell catalyst and sorbent of this invention includes a coreof reactive, but comparatively physically weak, material and a strongbut porous shell with catalytic properties. The shell maintains thestructural integrity of the catalyst and sorbent during its use. Thesorbent is also preferably able to retain its structural integrityduring numerous cycles of use and regeneration.

The core is made of any material which will absorb or react with carbondioxide (CO₂) in order to remove it from the H₂-containing product. Theshell of the sorbent is a material which gives the pellet sufficientstrength while allowing CO₂ gas to penetrate to (diffuse to) the sorbentmaterial in the core. With a calcium oxide core, the successfulprotective layer (shell) can be largely refractory cement or alumina.Preferred materials for the preparation of CaO cores include powderedlimestone, dolomite, and plaster of Paris.

Types I, III, and IV Portland cement, high alumina cement (HAC), andcalcium aluminate refractory cement can be used as shell materials.These are hydraulic cements which require curing. An alternative shellmaterial is comprised of a sinterable powder (e.g., alumina) which uponheat treatment forms a strong, porous shell. Examples of core and shellraw materials which are appropriate for use in this invention include,but are not limited to, the following: Core material: Shell material:CaO (lime & swollen lime) Bentonite clay CaCO₃ (limestone, swollenAttapulgite clay limestone) CaCO₃•MgCO₃ (dolomite) Zeolite material MgOPortland cements SrO High temperature refractory cement BaO Alumina Flyash Calcium aluminates Magnesium oxysulfate cement

It may also be possible to use CaO.MgO derived from dolomite as a corematerial for removing CO₂ from gas produced by gasifying coal. In thisrespect, it is believed the following reaction takes place:CaO.MgO_((s))+CO_(2(g))

CaCO₃.MgO_((s))

While MgO does not participate in the reaction, it serves to maintainthe structural integrity and chemical reactivity of the sorbent.

The core-in-shell concept is not limited to the specifically-listedmaterials above. The concept can be applied to other absorbent materialswhich lack the necessary strength and durability in themselves towithstand repeated loading and regeneration. The effects of loading andregeneration can be particularly severe where the sorbent is convertedfrom one chemical species to another during loading and then isconverted back to the original species during regeneration. Such changesare accompanied by changes in the physical structure of the materialwhich can produce changes in the specific volume of the material.Repeated swelling and shrinking of the material as it is loaded andregenerated can weaken the material and lead to spalling anddisintegration. This type of failure has been observed with zinc-basedsorbents for H₂S where the material undergoes significant changes inspecific volume during regeneration.

The core-in-shell concept is particularly advantageous for supportingany reactive sorbent which may lack the necessary strength to withstandextensive physical handling or repeated changes in specific volume as itis loaded and regenerated. The shell itself can be made of variousrefractory materials which do not react with the gases being treated.Other potential shell materials include calcium aluminate cement,aluminum oxide, and various aluminates and silicates or various types ofclay which are widely used for making ceramic and refractory products.One of ordinary skill in the art would be able to ascertain additionalcore or shell materials appropriate to their particular application.

A small amount of a pore-forming component, such as dry potato starch,can be mixed with the shell-forming material before being applied. Thestarch, or other pore-forming component, will decompose when the pelletsare heated to a high temperature, thereby increasing the porosity of theshell. An example of an alternative pore-forming component is granularpolyethylene glycol. Any material known to one of ordinary skill in theart which will form pores (i.e. decompose at hightemperature/temperature of use) without interfering with the purposes ofthis invention can be used.

An alternative pore-forming material is limestone itself which can beincorporated in the shell because heating the pellets to the temperatureof use will decompose the limestone in the shell and provide sufficientporosity.

Although increasing shell thickness can increase pellet strength, it canalso reduce the rate of diffusion through the shell and reduce sorbentcapacity. One of ordinary skill in the art can optimize the compositionfor adsorption capacity versus crushing strength for a desiredapplication.

Appropriate catalysts for use in the invention include any catalyst forthe steam reforming of hydrocarbons. The catalyst may also be one usedfor the water gas shift reaction. These two types of catalysts are wellknown in the art and may include one or more of the following metalsand/or oxides of the same: tungsten, cobalt, molybdenum, vanadium,potassium, lanthanum, iron, platinum, palladium, ruthenium, zinc,chromium, copper, or nickel. When catalytically reforming methane in thepresence of a sulfide impurity (for e.g., H₂S), it is believed thatcobalt and molybdenum are the preferred catalysts since, unlike nickel,they tend to withstand inactivation in the presence of large amounts ofsulfur compounds in the feed gas. If the feed gas is relatively pure,nickel is the preferred catalyst.

While various methods can be used for incorporating a catalyst-formingmaterial in the shell, one method is to mix such a material with theother shell-forming materials before they are applied. Another method isto impregnate core-in-shell pellets with the catalyst after the pelletshave been made. For example, a reforming catalyst can be prepared byimpregnating a porous shell with any of the metals listed above in saltform, such as oxide.

Although the catalyst/sorbents of this invention can be produced by anymethod which provides the desired physical and chemical characteristics,one of the preferred methods is as follows. The pellet cores areprepared by placing a measured amount of one or more powderedcore-forming materials in a revolving drum or inclined pan pelletizer.While in the pelletizer, the powder is moistened with a water spray thatcauses the core material to ball up into small pellets. The conditionscan be controlled to produce pellets of a desired diameter. The pelletsare sized by screening and those of an appropriate size are coated inthe next step with the shell-forming material.

In coating the pellet cores with the protective layer, the powderedmaterial for the protective layer, e.g., cement or alumina, is added tothe pelletizing drum or inclined pan pelletizer while the pellets arebeing sprayed at set intervals with water. In some cases, dilute ligninsolution may be included as a temporary binder in the coating process.The process can be carried out continuously by using two pelletizingdrums in series separated by a vibrating screen. The core formingingredients are supplied at a constant and controlled rate to the firstpelletizing drum, and as the spherical cores are formed, they aredischarged onto the vibrating screen. The material which passes throughthe screen is returned to the first pelletizing drum for repelletizing.The pellets that do not pass through the screen are conducted to thesecond pelletizing drum for coating with the shell-forming material.

The pellets are preferably comprised of cores ranging from about 3-8 mmin diameter and of shells ranging from about 0.3-1.0 mm in thickness.Pellets prepared for commercial application can be either smaller orlarger. One of skill in the art would be able to choose the core andshell dimensions that will work best for the particular application ofinterest.

Once coated, the pellets are dried and screened to provide pellets of auniform and appropriate size. Pellets coated with hydraulic cement areoptionally cured in a steam atmosphere at 100° C. for about 1-3 days. Inthe case of alumina-coated pellets, neither air drying nor steam curingis required. The pellets are calcined in air at about 1100° C. for 2hr., causing partial sintering of the shell material to produce a strongbut porous structure. Calcination also alters the core material bydecomposing CaCO₃ in the case of limestone cores or removing water ofcrystallization in the case of plaster of Paris cores. However, thecalcium sulfate present in plaster cores is left largely intact.

In order to convert CaSO₄ plaster of Paris cores to CaO, the pellets aretreated at 1050° to 1100° C. to several cycles of reduction andoxidation. During the reducing phase of each cycle, the pellets aretreated with a reducing gas, e.g. 10% H₂ or 30% CO in nitrogen, for 1 to3 minutes, and during the oxidizing phase the pellets are treated withan oxidizing gas, e.g. air, for 1 to 3 minutes.

There are several different appropriate methods that may be used toincorporate the shell material with a reforming catalyst depending onthe catalytic material selected. One method is the pore volumeimpregnation technique which is used to fill the shell pores with asaturated solution of the metal catalyst in salt form. The pellets arecontacted with sufficient solution only to the extent necessary tosaturate the porous shell. The pellets are then dried to remove thewater and to deposit the metal catalyst within the pores. Thisimpregnation process may be repeated until the desired level ofimpregnation is achieved. The pellets are next heated to a temperatureranging from about 300°-700° C., with a temperature of about 500° C.being preferred, in order to decompose the metal salt, therebyconverting it to the oxide form. The catalyst is subsequently activatedby an appropriate treatment. In the case of a nickel catalyst, thepellets are treated with hydrogen at about 300-500° C. to reduce thenickel oxide to its elemental metal form.

In a second method, prior to the impregnation method described above,the pellets are treated with carbon dioxide gas at 500-800° C. toconvert the CaO cores to CaCO₃ cores. The pellets are then cooled toambient temperature and the pore volume impregnation technique is usedto fill the shell pores with an aqueous solution of metal salt. Thepellets are subsequently dried to remove the water and to deposit themetal salt with the pores of the pellet. Again, multiple impregnationsmay be used to achieve the requisite amount of loading of the metalcatalyst. The pellets are then heated to decompose the metal salt andthereafter treated to activate the catalyst in the manner outlinedabove.

The hydrogen production process of this invention can employ either ofthe two types of primary chemical reactions, shown below:CH_(4(g))+2H₂O_((g))=CO_(2(g))+4H_(2(g))  (1)CO_((g))+H₂O_((g))=CO_(2(g))+H_(2(g))  (2)

Reaction (1) illustrates the reaction of methane with steam to producecarbon dioxide and hydrogen, and preferably employs a Ni catalyst.Reaction (2) illustrates the reaction of CO with steam to produce CO₂and H₂ which is known as the water gas shift reaction. This reactionpreferably uses an iron oxide catalyst. The sorbent of the inventionremoves carbon dioxide (CO₂) from the hot gas stream by means of thefollowing reaction (using CaO as an example):CaO_((s))+CO_(2(g))=CaCO_(3(s))  (3)

The combination of an absorbent core and a catalytic shell offers atleast two important advantages over prior art processes. First, thepresence of a strong absorbent for the carbon dioxide helps to driveeither reaction (1) or (2) which can be limited by thermodynamicequilibrium. Second, the heat absorbed by the highly endothermicreactions (1) and (2) is largely offset by the heat generated byreaction (3). Therefore, there is little need to either add or removeheat from the reaction as a whole, which greatly simplifies the designof the reaction system and improves the overall efficiency and economicsof the process.

The following examples are offered to illustrate but not limit theinvention. Thus, they are presented with the understanding that variousformulation modifications as well as method of delivery modificationsmay be made and still be within the spirit of the invention.

EXAMPLE 1

Pore Volume Impregnation Technique Using Nickel

An alcohol solution of nickel salt is prepared by dissolving eithernickel acetate, Ni(C₂H₃O₂).4H₂O, or nickel nitrate, Ni(NO₃)₂.6H₂O, in95% ethanol (5% water) to form a saturated solution of the salt.Core-in-shell pellets are contacted with a limited quantity of solution,then dried to remove the solvent and to leave the salt behind in thepores. The process of impregnation can be repeated several times toachieve the desired nickel loading in the pellets. The pellets are thenheated to 500° C. to decompose the nickel salt and thereby convert itinto nickel oxide. The pellets are subsequently treated with hydrogen at300-500° C. to reduce the nickel oxide to nickel.

EXAMPLE 2

Pore Volume Impregnation Technique Using Nickel and CaO Pellet CoresWith Carbon Dioxide Pretreatment

Prior to impregnating the core-in-shell pellets with catalyst, thepellets are treated with carbon dioxide gas at 500 to 800° C. to convertthe CaO cores to CaCO₃ cores. The pellets are cooled to ambienttemperature and the pore volume impregnation technique described inExample 1 is used to fill the shell pores with an aqueous solution of anickel salt. A solution containing 10-30 wt. % Ni(NO₃)₂ is suitable forthis purpose. The pellets are subsequently dried to remove the water andto deposit the nickel salt within the pore. Multiple impregnations canbe used to achieve the desired nickel loading. The pellets are thenheated to 500° C. to decompose the nickel salt and further heated to900° C. to convert CaCO₃ in the core material to CaO. The pellets aresubsequently treated with hydrogen at 300-500° C. to reduce the nickeloxide to nickel.

It should be appreciated that the compositions and methods of thisinvention may be extended to other chemical reaction systems whichrequire a solid catalyst and where it is advantageous to separate theproducts of reaction by selective absorption of a reaction product.Also, modifications of the composition and the ranges expressed hereinmay be made and still come within the scope and spirit of the presentinvention.

Having described the invention with reference to particularcompositions, theories of effectiveness, and the like, it will beapparent to those of skill in the art that it is not intended that theinvention be limited by such illustrative embodiments or mechanisms, andthat modifications can be made without departing from the scope orspirit of the invention, as defined by the appended claims. It isintended that all such obvious modifications and variations be includedwithin the scope of the present invention as defined in the appendedclaims. The claims are meant to cover the claimed components and stepsin any sequence which is effective to meet the objectives thereintended, unless the context specifically indicates to the contrary.

All articles cited herein and in the following list are hereby expresslyincorporated in their entirety by reference.

CITATIONS

-   Akiti, Jr., T. T., et al. A regenerable calcium-based core-in-shell    sorbent for desulfurizing hot coal gas. Ind. Eng. Chem. Res. 2002,    41, 587-597.-   Balasubramanian, B. et al. Hydrogen from methane in a single-step    process. Chem. Eng. Sci. 1999, 54, 3543-3552.-   Ding, Y. et al. Adsorption-enhanced steam-methane reforming. Chem.    Eng. Sci. 2000, 55, 3929-3940.-   Hufton, J. R. et al. Sorption-enhanced reaction process for hydrogen    production. AIChE J., 1999, 45, 248-256.

1. A method of manufacturing a composition for producing hydrogen fromindustrial gas-phase chemical reactions comprising: forming an interiorcore that reacts with or absorbs CO₂; coating the core with a protectiveshell; and incorporating a catalyst into the protective shell.
 2. Themethod of claim 1 wherein the interior core is formed by pelletizing acore-forming material selected from the group consisting of limestone,lime, dolomite, plaster of Paris, anhydrite, and gypsum.
 3. The methodof claim 2 further including the step of drying and screening thecore-forming material to form pellets of a uniform size following thecoating step.
 4. The method of claim 1 whereby the shell is comprised ofhydraulic cement, and the method further includes the step of curing thecore following the coating step in a steam atmosphere at 100° C. forabout 1-3 days.
 5. The method of claim 1 whereby the shell is comprisedof a material selected from the group consisting of alumina, limestone,and plaster of Paris, and the method further includes the step ofcalcining the core following the coating step to cause partial sinteringof the shell material.
 6. The method of claim 1 whereby the catalyst isincorporated into the shell by impregnating the shell with a solution ofthe catalyst.
 7. The method of claim 6 whereby the impregnation step isrepeated at least once.
 8. The method of claim 1 whereby the core istreated with carbon dioxide at 500-800° C. following the coating stepand prior to the catalyst incorporation step.